Microwave integrated circuits (“MICs”) typically consist of a number of separate high-frequency components, such as thin-film or thick-film transmission lines, semiconductor integrated circuits (“ICs”), active or passive electronic devices, and printed wiring boards (“PWBs”). MICs may also include surface-mount devices, integrated subcomponents, and grounding structures. The devices are electrically interconnected using a variety of techniques.
Microstrip and coplanar transmission lines are examples of planar transmission lines often used in MICs. In a microstrip transmission line, a center conductor is separated from a ground plane by a selected thickness of dielectric material to obtain a characteristic impedance of the transmission line. Fifty ohms is an example of a characteristic impedance often used in MICs. In a coplanar transmission line, ground planes extend along each side of a center conductor to obtain a characteristic impedance. In both cases, center conductors of adjacent transmission lines are typically connected together using one of four bonding methods: 1) a single wire bond between the center conductors of each transmission line, 2) parallel wires bonded symmetrically between the center conductors of each transmission line, 3) mesh bonding between the center conductors of each transmission line, and 4) ribbon bonding between the center conductors of each transmission line. Each of these bonding methods is also used to connect planar transmission lines to other devices.
At microwave frequencies, a single wire bonded between center conductors of adjacent transmission lines results in a signal discontinuity. In other words, the desired characteristic impedance is not maintained across the bond wire. The impedance of the bond wire is usually higher than 50 ohms, which is frequently the characteristic impedance of the transmission lines. The discontinuity leads to scattering and reflections of the incident signal, which degrades the performance of the MIC. Multiple bond wires create a similar discontinuity, but not as severe as a single bond wire.
Mesh and ribbon bonds have a lower impedance than a single bond wire or even multiple bond wires of similar length. However, mesh and ribbon bonds still present an impedance discontinuity. Unfortunately, mesh and ribbon bonds are more difficult to fabricate than wire bonds. Mesh bonding is a manual bonding process that uses fragile pieces of fine metal mesh that must be carefully handled by a skilled operator to avoid tearing. Ribbon bonding can be manual or automatic. Manual ribbon bonding can be as difficult as mesh bonding. Automatic ribbon bonding cannot easily adjust the ribbon width, and is not as flexible of a manufacturing process as wire bonding.